Carbon Solubility in the CaO-SiO₂-3MgO-CaF₂ Slag System

The carbon solubility in the CaO-SiO₂-3MgO-CaF₂ slag system at 1773 K (1500 °C) was investigated under CO/Ar and CO/N₂ gases. Higher extended basicity [(CaO + MgO)/SiO₂) increased the carbon solubility in the slag as the activity of free oxygen ions (\( \varvec{a}_{{{\mathbf{O}}^{2 - } }} \)] promoted the reaction of the free carbide mechanism. Higher CaF₂ also resulted in higher carbon dissolution into the slag as fluorine ions interact with the bridged oxygen (O⁰) in the melt structure to increase the activity of the free oxygen ions in the melt. Structural information obtained from the Fourier transformed infra-red (FT-IR) and Raman revealed a depolymerization of the network structure as the simpler structural units of NBO/Si = 4 increased and the Si-O-Si bending vibrations decreased with higher basicity and CaF₂ content. This correlated well with higher free oxygen ions (O²⁻) in the slag system and subsequently higher carbon dissolution. A correlation of the theoretical optical basicity (Λ_th) with the logarithm of the carbon content in slag showed a relatively similar trend and an increase of carbon was observed with higher optical basicity.